1. Field of the Invention
The present invention relates generally to a microprocessor based control system for a hydronic heating system. More particularly, the subject invention pertains to a microprocessor based control system which is heuristic in nature, and controls the boiler and circulation of a heating system in accordance with presently sensed heating demands and the immediate past operating history of the heating system to control the operation thereof in an extremely efficient manner and thereby minimize operational costs.
2. Discussion of the Prior Art
In many conventional heating systems in the prior art an enclosed structure such as a building is heated by hot water circulated through a heat exchanger in the structure. The water is heated in a boiler having a burner in which 0 oil, gas or another fuel is burned, and a circulator pump circulates water between the boiler and the heat exchanger. During a heating season, the water in the boiler is often maintained between a minimum preselected temperature, usually around 160.degree. F., and a maximum preselected temperature, typically around 190.degree. F. When the structure is heated, the requisite heat is supplied via the heat exchanger from the circulating hot water which is cooled as a consequence thereof. The cooled water is circulated back to the boiler for reheating, and additional heat is supplied to the boiler from the furnace. Typically, a thermostat in a heating zone of the structure is used to initiate circulation and turn on the burner when calling for heat and to turn off circulation and the burner when there is sufficient heat in the region to satisfy the thermostat. The water in the boiler generally cools down slowly to the minimum temperature, and the boiler is then fired to reheat the water to the maximum temperature. In these systems a substantial amount of energy is often wasted by maintaining the boiler water within the preset temperature range regardless of the heating requirements of the buildings. Moreover, substantial quantities of heat are also lost up the flue because the furnace is continually recycled, and also abeyant heat remaining in the boiler and distribution pipes after a thermostat is satisfied is also generally wasted.
Keeney U.S. Pat. 4,108,375 is considered to be pertinent to the present invention, and discloses a boiler control arrangement in which the temperature of the boiler water is maintained just hot enough to provide sufficient heat for the enclosed building structure. This is accomplished by sensing the boiler water temperature and the outside ambient temperature, making a comparison between the two, and then controlling the operation of the boiler and the circulators based on the results of this comparison. This system controls both the boiler water temperature and the circulators which control the circulation of the boiler water through the system. As a zone thermostat within the enclosed building calls for heat, its respective circulator is turned on, and any one or all of the circulators may be operated at the same time. However, the circulator for the last zone to call for heat continues to run, even though its zone is not calling for heat, until another zone thermostat calls for heat. At least one circulator is kept running as long as the boiler water temperature is higher than a predetermined temperature. This predetermined temperature is chosen depending on the nature of the heating season in the area where the control device is employed. Thus, when the temperature of the boiler water drops to the predetermined temperature, all pumps are cut off. This will occur at the end of a heating season, while at the beginning of the next heating season, the first time that there is a demand for heat, the boiler is refired and when the boiler water temperature rises to the predetermined temperature the circulator pumps are again enabled to run.
The Keeney system effects a savings in the energy required to heat an installation by providing a more efficient control system than a standard heating arrangement. As each of the heated zones of the installation demands heat, the respective circulators are started or zone control valves are opened to permit the boiler water to flow through the zones being heated. As the demand is satisfied in each of the zones, the control valves or circulators are shut off. However, the heat is continued to be supplied to the last zone to call for it. By continuing to circulate the heating medium to the last zone to call for heat, a substantial amount of the heat that was put into the boiler is extracted for use in heating the installation.
However the Keeney control system suffers from a number of drawbacks, and is not capable of automatically modifying its target temperature once it has been manually set. The circulation control in the Keeney system is similar to normal thermostat control systems in that temperature satisfaction of the thermostat discontinues circulation unless it is the last zone to call for heat. This does not accomodate varying circulation periods based upon the different heating demands of the zones. Also, where domestic hot water is obtained directly from the boiler (tankless coil systems), no accomodation is made to permit the boiler to heat the water to the higher temperature required for domestic hot water.